Abstract

Background: In nano-size α-Fe₂O₃ particles, the Morin transition temperature was reported to be suppressed. This suppression of the TM in nano-size α-Fe₂O₃ was suggested to be due to high internal strain and to the enhanced role of surface spins because of the enhanced surface to volume ratio. It was reported that for nanoparticles of diameters less than 20 nm, no Morin transition was observed and the antiferromagnetic phase disappears. In addition, annealing of samples was reported to result in both an increase of TM and a sharper transition which were attributed to the reduction in defects, crystal growth, or both.

Objective: In this work, we investigated the role of applied magnetic field in TM, the extent of the Morin transition, thermal hysteresis, and the spin-flop field in synthetic α-Fe₂O₃ nanoparticles of diameter around 20 nm.

Methods: Hematite nanoparticles were synthesized using the sol-gel method. Morphology and structural studies of the particles were done using TEM, and XRD, respectively. The XRD patterns confirm that the particles are hematite with a very small maghemite phase. The average size of the nanoparticles is estimated from both TEM images and XRD patterns to be around 20 nm. The magnetization versus temperature measurements were conducted upon heating from 5 K to 400 K and cooling down back to 5 K at several applied fields between 50 Oe and 500 Oe. Magnetization versus magnetic field measurements between -5 T and +5 T were conducted at several temperatures in the temperature range of 2-300 K.

Results: We report three significant findings in these hematite nanoparticles. Firstly, we report the occurrence of Morin transition in hematite nanoparticles of such size. Secondly, we report the slight field dependence of Morin transition temperature. Thirdly, we report the strong temperature dependence of the spin-flop. Zero-field-cooled magnetization versus temperature measurements were conducted at several applied magnetic fields.

Conclusion: From the magnetization versus temperature curves, Morin transition was observed to occur in all applied fields at Morin transition temperature, TM which is around 250 K with slight field dependence. From the magnetization versus magnetic field curves, spin-flop in the antiferromagnetic state was observed and found to be strongly temperature dependent. The results are discussed in terms of three components of the magnetic phase in our sample. These are the paramagnetic, soft ferromagnetic, and hard ferromagnetic components.

Keywords: Morin transition, soft ferromagnetic, ZFC, spin canting, exchange bias, nanoparticles.

Graphical Abstract